The purpose of this project was to establish whether mature mouse oocytes can repair damage caused by agents known to cause lesions in DNA and to assess the effects of these agents on the developmental process. Our results have shown that mature mouse oocytes are capable of performing DNA repair synthesis. However, in spite of this capability, one-cell embryos are very sensitive to low doses of UV light and ionizing radiation. None of the one-cell embryos developed past the two-cell stage. Both forms of radiation greatly accelerated fragmentation and degeneration of the cells. However, treatment of one-cell embryos with the carcinogen, NA-AAF, only reduced the number of embryos that developed to the blastula stage and did not increase the rate of fragmentation or degeneration. In contrast, exposure of two-cell embryos to low doses of UV light, ionizing radiation, or NA-AAF did not block development to the blastula stage. Our findings suggest that a target for radiation other than DNA exists in one-cell embryos and this target apparently is necessary for first cleavage to occur.